Diffractive Electroproduction of rho and phi Mesons at HERA

TL;DR

This paper reports on measurements of diffractive electroproduction of rho and phi mesons at HERA, revealing a transition to hard scattering behavior at higher Q^2 and providing new insights into diffraction dynamics within QCD.

Contribution

It presents the first detailed measurements of spin density matrix elements and cross section ratios in diffractive meson production, supporting models of QCD diffraction mechanisms.

Findings

Transition to hard scattering behavior at Q^2 > 10-20 GeV^2

Support for flavour independence and proton vertex factorisation

New measurements of spin density matrix elements and amplitude ratios

Abstract

Diffractive electroproduction of rho and phi mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb^-1. About 10500 rho and 2000 phi events are analysed in the kinematic range of squared photon virtuality 2.5 < Q^2 < 60 GeV^2, photon-proton centre of mass energy 35 < W < 180 GeV and squared four-momentum transfer to the proton |t| < 3 GeV^2. The total, longitudinal and transverse cross sections are measured as a function of Q^2, W and |t|. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small q\bar{q} dipoles, for Q^2 larger than 10 to 20 GeV^2. They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q^2 + M_V^2)/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.